A device (e.g. welding machine) that discharges arc between an electrode and a processed object (referred to as parent material, hereinafter) to melt the parent material for processing typically includes a power control circuit for controlling an output current flowing between the electrode and the parent material or output voltage applied between them.
In recent years, such a power control circuit has been usually formed of an inverter circuit including a high-speed switching element and a power conversion transformer, becoming widely used as an inverter-controlled welding machine.
Such an inverter-controlled welding machine typically includes a full-bridge inverter circuit. The welding machine drives a power semiconductor element (e.g. IGBT and MOSFET) composing a bridge circuit, at an inverter frequency (usually from several kHz to approximately 100 kHz). Simultaneously, the welding machine compares an output current to an output current set value (or output voltage to an output voltage set value) to control conduction time of the power conversion transformer, thereby obtaining output with current or voltage characteristics preferable for welding output power.
As full-bridge inverter control method, some conventional inverter-controlled welding machines use pulse-width modulation (referred to as PWM hereinafter), which controls conduction time of a switching element. Others use phase control method (also referred to as phase shift method), which controls conduction timing of a switching element (refer to patent literature 1).
Further, others can use a method in which features of PWM and phase control method are merged; one bridge circuit out of the two is controlled with a fixed conduction width; and the other undergoes pulse-width modulation (referred to as one-side bridge fixed conduction width PWM control, hereinafter).
Hereinafter, a description is made of welding machines by the three methods: PWM, phase control, and one-side bridge fixed conduction width PWM control.
First, PWM above is described using FIG. 11.
FIG. 11 shows an outline structure of substantial parts of an arc welding machine including an inverter control circuit by conventional PWM.
In FIG. 11, first rectifier 5 rectifies three- or single-phase AC input. First switching element 1 and second switching element 2 convert output from first rectifier 5 to an alternating current. Second rectifier 7 rectifies output from power conversion transformer 6. Output current detector 8 detects an output current. Current detecting part 9 converts a signal from output current detector 8 to a feedback signal. Output power setting part 12 is provided to preliminarily set average and effective values during a predetermined period, of a welding current or welding voltage as output from the welding machine. Error amplification part 11 determines an error between a signal output from current detecting part 9 and a signal set by output power setting part 12, and amplifies the error. Inverter driving basic pulse generating part 13 generates a driving waveform fundamental for inverter control. Pulse-width modulating part (PWM part, hereinafter) 14 outputs a control signal for controlling conduction widths of switching elements 1 and 2 according to an error amplification signal from error amplification part 11. Driving circuits 21, 22, 23, and 24 convert the control signal to a drive signal for driving switching elements 1 and 2 according to a signal output from pulse-width modulating part 14, and outputs the drive signal. Here, inverter control part 29 enclosed by the dashed-dotted line includes inverter driving basic pulse generating part 13 and pulse-width modulating part 14.
To control output power of a non-consumable electrode arc welding machine (e.g. TIG welding machine), current control is usually performed in which an output current is made equal to a current set value. To control output power of a consumable electrode arc welding machine (e.g. MAG welding machine), meanwhile, voltage control is performed in which output voltage is made equal to a voltage set value. The operation principles of an inverter used for output control of the above-described arc welding machines are the same, and thus a description is made of current control (controlled for a constant current value) as an operation example of an inverter.
Three- or single-phase AC input rectified by first rectifier 5 is converted to an alternating current with a high frequency by a full-bridge inverter circuit composed of switching elements 1, 2, 3, and 4, and then is input to the primary side of transformer 6. Here, switching elements 1 and 2 compose first switching circuit 25, and switching elements 3 and 4 compose second switching circuit 26. The secondary-side output of transformer 6 is rectified by second rectifier 7 and is supplied to an electrode and parent material (both are arc loads, not shown) through output terminals 38 and 39.
An output current from the welding machine is detected by output current detector 8, and a detection signal proportional to the output current is input to error amplification part 11 from output current detector 8 through current detecting part 9. Error amplification part 11 compares an output power set value from output power setting part 12 to a current signal from current detecting part 9, and outputs an error amplification signal between both. The error amplification signal is converted by pulse-width modulating part 14 to driving pulses with a width corresponding to the magnitude of the error amplification signal on a basis of a basic pulse waveform for inverter driving generated by inverter driving basic pulse generating part 13.
The driving pulses are separated one by one alternately into two series to become 2-series drive signals for inverter driving. One series is input to driving circuits 21 and 24 as a signal for driving switching elements 1 and 4 simultaneously; the other is input to driving circuits 22 and 23 as a signal for driving switching elements 2 and 3 simultaneously.
These drive signals are converted to those suitable for driving switching elements 1 to 4 by respective driving circuits 21 to 24, and are input to switching elements 1 to element 4.
As a result that switching elements 1 and 4; and switching elements 2 and 3 simultaneously conduct alternately, output from first rectifier 5 is converted to an alternating current. The alternating current is input to the primary winding of transformer 6; converted to output power suitable for welding; and output from the secondary winding of transformer 6. Output from the secondary winding of transformer 6 is converted to a direct current by second rectifier 7 and is output from the welding machine as welding output power.
Here, error amplification part 11 has an amplification factor as high as 100 times to 1,000 times for example. This allows maintaining constant current characteristics according to an output current set value even for a change in output voltage due to a change in load condition of output.
A description is made of an operation example of a welding machine by PWM later using FIG. 14.
Next, a description is made of the above welding machine by phase control method using FIG. 12.
FIG. 12 shows an outline structure of substantial parts of an arc welding machine including an inverter control circuit by conventional phase control method. In the following drawings, the same component is given the same reference mark, and its description may be omitted.
In FIG. 12, phase control part 15 outputs a control signal for controlling conduction of switching elements 1 to 4 according to an error amplification signal from error amplification part 11.
Three- or single-phase AC input rectified by first rectifier 5 is converted to an alternating current with a high frequency by a full-bridge inverter circuit composed of switching elements 1, 2, 3, and 4, and then is input to the primary side of transformer through capacitor 10. The secondary-side output of transformer 6 is rectified by second rectifier 7 and is supplied to an electrode and parent material (both are arc loads, not shown) through output terminals 38 and 39.
An output current from the welding machine is detected by output current detector 8, and a detection signal proportional to the output current is input to error amplification part 11 from output current detector 8 through current detecting part 9. Error amplification part 11 compares an output power set value from output power setting part 12 to a signal from current detecting part 9, and outputs an error amplification signal between both. The error amplification signal is converted by phase control part 15 to driving pulses with a phase difference corresponding to the level (magnitude) of the error amplification signal on a basis of a basic pulse waveform for inverter driving generated by inverter driving basic pulse generating part 13.
Inverter driving basic pulse generating part 13 outputs inverter driving basic pulse for driving first switching element 1 and second switching element 2 composing first switching circuit 25 alternately with a fixed conduction width. Here, first switching circuit control part 27 has inverter driving basic pulse generating part 13 to control first driving circuit 21 and second driving circuit 22. Second switching circuit control part 28 has phase control part 15 to control third driving circuit 23 and fourth driving circuit 24. The inverter driving basic pulses are converted to a signal suitable for driving switching elements 1 and 2 by driving circuits 21 and 22, and is input to switching elements 1 and 2.
A phase control signal generated by phase control part 15 works for outputting driving pulses for alternately driving third switching element 3 and fourth switching element 4 composing second switching circuit 26 with a phase difference corresponding to an error amplification signal in relation to operation of first switching circuit 25. These drive pulses are converted to a signal suitable for driving switching elements 3 and 4 by driving circuits 23 and 24 and are input to switching elements 3 and 4.
Then, during a period when a conduction period of switching element 1 coincides with that of switching element 4, a primary current flows through transformer 6 from first switching element 1 to fourth switching element 4. Meanwhile, during a period when a conduction period of switching element 2 coincides with that of switching element 3, a primary current flows through transformer 6 from third switching element 3 to second switching element 2. In this way, output from first rectifier 5 is converted to an alternating current; is converted to output power suitable for welding; and is output from the secondary winding of transformer 6. Output from the secondary winding of transformer 6 is converted to a direct current by second rectifier 7 and is output from the welding machine as welding output power.
Here, error amplification part 11 has an amplification factor as high as 100 times to 1,000 times, which allows maintaining constant current characteristics corresponding to an output current set value even for a change in output voltage due to a change in load condition of output.
An operation example of a welding machine by phase control method is described later using FIG. 15.
Next, a description is made of the above welding machine by one-side bridge fixed conduction width PWM control method using FIG. 13.
FIG. 13 shows an outline structure of substantial parts of an arc welding machine including an inverter control circuit by conventional one-side bridge fixed conduction width PWM control method.
FIG. 13 shows the configuration of FIG. 12 in which PWM part 14 is substituted for phase control part 15. Hereinafter, the operation is described.
Inverter driving basic pulse generating part 13 outputs inverter driving basic pulses for driving first switching element 1 and second switching element 2 composing first switching circuit 25 alternately with a fixed conduction width. The inverter driving basic pulses are converted to a signal suitable for driving switching elements 1 and 2 by driving circuits 21 and 22, and the signal is input to switching elements 1 and 2.
The error amplification signal input from error amplification part 11 is converted by PWM part 14 to driving pulses with a width corresponding to the level (magnitude) of the error amplification signal on a basis of a basic pulse waveform for inverter driving generated by inverter driving basic pulse generating part 13. The driving pulses are input one by one alternately to driving circuits 23 and 24 as a signal for driving third switching element 3 and fourth switching element 4.
Then, during a period when a conduction period of switching element 1 coincides with that of switching element 4, a primary current flows through transformer 6 from first switching element 1 to fourth switching element 4. Meanwhile, during a period when a conduction period of switching element 2 coincides with that of switching element 3, a primary current flows through transformer 6 from first switching element 3 to second switching element 2. In this way, output from first rectifier 5 is converted to an alternating current; is converted to output power suitable for welding; and is output from the secondary winding of transformer 6. Output from the secondary winding of transformer 6 is converted to a direct current by second rectifier 7 and is output from the welding machine as welding output power.
An operation example of the above welding machine by one-side bridge fixed conduction width PWM control method is described later using FIGS. 16A through 16C
Next, a description is made of the above welding machine that exercises control by the three types of methods using FIGS. 14A through 14C, 15A through 15C, and 16A through 16C.
FIGS. 14A through 16C are schematic diagrams showing operation of an inverter of an arc welding machine including a conventional inverter control circuit. FIGS. 14A through 14C show operation by PWM method; FIGS. 15A through 15C, by phase control method; and FIGS. 16A through 16C, by one-side bridge fixed conduction width PWM control method.
FIGS. 14A, 15A, and 16A show operation states at low output (i.e. short inverter conduction period); FIGS. 14B, 15B, and 16B, at middle output (i.e. middle-range inverter conduction period); and FIGS. 14C, 15C, and 16C, at high output (i.e. long inverter conduction period). FIGS. 14A through 16C schematically show conduction states of first switching element 1 through fourth switching element 4, conduction periods of the inverter circuit; and waveforms of a primary current through transformer 6.
In FIGS. 14A through 16C, a part indicated by an arrow, of an operation waveform of first switching element 1 to fourth switching element 4 shows how the waveform changes during output control. An arrow appended at the falling edge of a waveform shows that the edge moves back and forth, and the waveform expands and contracts to change the conduction period. An arrow appended at the top of a waveform shows that the waveform does not expand or contract, the conduction period does not change, and the entire waveform moves back and forth on along the time axis. This indicates that the phase of a waveform changes to control output as shown by the inverter conduction period. A horizontally striped part of the waveform of a primary current through a transformer represents a regenerative current.
First, a description is made of an operation example of a welding machine by PWM method using FIGS. 14A through 14C. FIG. 14A shows operation at low output, where the switching element does not conduct (a transformer current is not flowing) due to such as delay operation (described later) of the driving circuit during minimum power output. FIG. 14B shows an operation example at middle output; and FIG. 14C, at high output. Both first switching circuit 25 and second switching circuit 26 are operating with PWM method.
Here, a description is made of the following situation using FIGS. 10A and 10B. That is, a switching element does not conduct due to such as delay operation of the driving circuit during minimum power output; a transformer current does not flow; and a transformer current becomes unstable near the minimum conduction width.
FIGS. 10A and 10B are schematic diagrams showing waveforms at some points of a switching element and a driving circuit, particularly for a combination of switching element 3 and driving circuit 23 out of the four switching elements and four driving circuits shown in FIG. 11. FIG. 10A shows an outline structure of driving circuit 23 using pulse transformer 31. FIG. 10B shows current waveforms at points A through C shown in FIG. 10A.
Driving circuit 23 shown in FIG. 10A is one including third switching element 3, inverter control part 29, pulse transformer operating transistor 30, pulse transformer 31, gate resistance 32, and capacitance 33 inside the gate of third switching element 3.
According to FIG. 10A, a drive signal output from inverter control part 29 is delayed at transistor 30 and pulse transformer 31 composing above-described driving circuit 23. Therewith, the signal is deformed by gate resistance 32 and capacitance 33 inside the gate of third switching element 3. In other words, as shown in FIG. 10B, the waveform at point A enters a state of delayed and reduced conduction time at point C where operation of third switching element 3 is shown. Accordingly, conduction (i.e. a flow of a transformer current) becomes unstable when the conduction time approaches the minimum conduction width, which sometimes causes a transformer current not to flow.
Next, a description is made of an operation example of a welding machine by phase control method using FIGS. 15A through 15C. FIGS. 15A through 15C show operation examples of an arc welding machine including an inverter control circuit by conventional phase control method. In all the areas of FIGS. 15A, 15B, and 15C, first switching circuit 25 shown in FIG. 12 is operating with a predetermined conduction width, and second switching circuit 26 is operating while undergoing phase control on first switching circuit 25. When second switching circuit 26 becomes nonconducting in this situation, a transformer current ceases to flow. Consequently, second switching circuit 26 interrupts a transformer current and first switching circuit 25 does not, thereby preventing heat generation caused by switching.
However, the large area size of the waveform indicated by the horizontal stripes in the waveform of a transformer current brings about a large regenerative current, thereby causing the regeneration diode of the switching element to generate more heat.
Here, a description is made of a regenerative current in phase control method using FIGS. 8A and 8B.
FIGS. 8A and 8B show changes in operating state of the inverter of a welding machine according to conventional phase control method. FIG. 8A shows the entire waveform for one cycle. FIG. 8B shows a conduction state of the switching element and a circuit current for periods indicated by T1 through T5 in FIG. 8A.
In FIG. 8A, L1 (the part surrounded by the oval solid line) indicates that switching loss is generated; L2 (the part surrounded by the oval broken line), is not generated. According to FIG. 8A, first switching element 1 indicated by Q1 does not interrupt a transformer current, and thus a conventional turn-off power loss is not generated. As indicated by T3 in FIG. 8B, however, first switching element 1 (indicated by Q1) and third switching element 3 (indicated by Q3) are in a conduction state for a long time, which causes a regenerative current to flow for a long time. Since this regenerative current is interrupted, a regeneration turn-off power loss is generated.
Next, a description is made of an operation example of a welding machine by one-side bridge fixed conduction width PWM control method using FIGS. 16A through 16C.
FIGS. 16A through 16C show operation examples of an arc welding machine including inverter control part 29 by conventional pulse-width modulation with one-side bridge fixed conduction width. FIG. 16A shows operation at low output, where the third and fourth switching elements do not conduct (a transformer current is not flowing) due to delay operation of the driving circuit during minimum power output. FIG. 16B shows operation at middle output; and FIG. 16C, at high output. Second switching circuit 26 shown in FIG. 13 is operating with PWM method in relation to first switching circuit 25. At this moment, second switching circuit 26 interrupts a transformer current and first switching circuit 25 does not interrupt, thereby preventing heat generation caused by switching.
Here, a description is made of the path of a charging current for a capacitor of a snubber in one-side fixed conduction width PWM method.
FIGS. 9A and 9B schematically show a charging current path of a snubber capacitor for a switching element, near a minimum transformer current. FIG. 9A shows operation of phase control method; and FIG. 9B, of one-side fixed conduction width PWM method.
In FIG. 9A, first switching element 1 and third switching element 3 are in a conduction state near a minimum current by phase control method. Accordingly, a charging current to second snubber capacitor 36 flows from first rectifier 5 to second snubber capacitor 36 through first switching element 1. A charging current to fourth snubber capacitor 37 flows from first rectifier 5 to fourth snubber capacitor 37 through third switching element 3. Accordingly, voltages at both ends of transformer 6 become nearly equal, and thus a charging current does not flow through transformer 6. Here, second snubber resistance 34 and fourth snubber resistance 35 are connected in parallel with transformer 6 placed therebetween.
In FIG. 9B, meanwhile, only first switching element 1 becomes in a conduction state near a minimum current by one-side fixed conduction width PWM method. Accordingly, both charging currents to second snubber capacitor 36 and fourth snubber capacitor 37 flow through first switching element 1, which causes the charging currents to flow through transformer 6. A current flowing through transformer 6 thus causes unintended output at the secondary side of transformer 6.
The above-described pulse-width modulation is performed in an inverter-controlled welding machine by conventional PWM method and by one-side bridge fixed conduction width pulse-width modulation. An attempt to exercise control with an inverter conduction width of a minute (approximately 1 μs) pulse width causes delay time in the drive path between inverter control part 29 and a switching element, particularly, delay time in the driving circuit and operation delay time in the switching element. Consequently, the switching element cannot be driven, or highly accurate control cannot be exercised in practice.
At this point, as shown in FIGS. 10A and 10B, a drive waveform signal output from inverter control part 29 activates switching element 3 through points A and B shown in FIG. 10A. On this occasion, however, the waveform at each point is deformed as shown in FIG. 10B due to delay operation in circuit components of driving circuit 23 and gate input capacitance 33 of third switching element 3. As shown in FIG. 10B, the conduction waveform of third switching element 3 at point C is not only delayed but is shortened in conduction width compared to the waveform at point A. Then, as shown in FIG. 10C, switching element 3 ceases to conduct as the drive signal width from inverter control part 29 becomes narrower.
This state is one such that a switching element is not conducting at minimum output in FIG. 14A showing an operation example of a welding machine by PWM control. Such a situation is of a problem particularly when requiring stable control on an output current in a range of several amperes, as in a TIG welding machine.
This phenomenon undesirably causes heat generation of an element and transformer saturation due to an unstable transformer current near a minimum drive width because the switching element is inadequately driven due to insufficient power for driving the gate of the switching element.
As shown in FIG. 9B, in operation by one-side bridge fixed conduction width pulse-width modulation near a minimum current, a charging current to a snubber capacitor causes a primary current to flow through transformer 6, thereby generating unintended output at the secondary side of the transformer. Accordingly, a large capacitance of the snubber capacitor leads to difficulty in control at low output, which prevents an output current or output voltage of the welding machine from falling to a minimum output.
An inverter-controlled welding machine by conventional phase control method does not need to expand and contract the driving pulse width of a switching element, and thus is not affected by delay time in the drive path, allowing control with a high degree of accuracy even at low output.
In the above case, however, the switching elements composing first switching circuit 25 and second switching circuit 26 simultaneously conduct for a relatively long time. This brings about a large regenerative current, thereby causing more heat generation in a regeneration diode contained in a switching element and a higher switching loss at the transistor.
As described above, phase control method involves a large regenerative current and difficulty in preventing heat generation in the device. Meanwhile, PWM control method has difficulty in controlling a minute current well accurately.